Rollers have been used for many years to convey webs through machines in a wide variety of processes, for example, in photographic film sensitizing, magnetic film coating, and the manufacture of plastic films of polyethylene and poly(ethylene terephthalate). Webs having a thickness above about 0.05 mm can be conveyed without wrinkles or creases by well-known contact roller conveyance technology. By "wrinkles" is meant longitudinally-oriented, smooth, transverse undulations in a web which are evoked by tension imbalances and which are not permanent deformations. By "creases" is meant longitudinally-oriented, abrupt, permanent deformations in a web. Webs having thicknesses above about 0.05 mm typically have sufficient beam or transverse compressional strength that they can adjust laterally on a roller surface to local transverse tension imbalances and therefore will pass conformably over the roller surface. These webs are said to be competent.
Demand for, for example, higher film and magnetic volumetric data storage densities and smaller electrical capacitors has led to a need for thinner and thinner webs, and such webs tend to be significantly less competent. Roller conveyance machines inherently exert more longitudinal tension than transverse tension on a web, and less competent webs respond by wrinkling longitudinally. The higher the discrepancy between longitudinal and transverse tension, the greater will be the severity of the wrinkles. Moderate undulations in unsupported web spans between rollers are not a problem in and of themselves except that they may cause the apparent width of the web to decrease, leading to web guiding or tracking problems. Serious problems can arise, however, when wrinkles come into contact with the cylindrical surface of a roller or a winding roll of web. The transverse cylindrical bending of the web will promote flattening of wrinkles, but frictional forces between the web and the roller resist flattening. If a wrinkle does not flatten laterally but instead folds over on itself because the compressional strength of the web is inadequate to overcome the lateral frictional resistance, the web is said to be incompetent. The web will become creased and typically will be permanently deformed in an irregular longitudinal pattern. Functional layers coated on the web can be disturbed or damaged by web creasing, either prior to or after coating. A crease which is wound into a winding roll can deform many convolutions previous or subsequent to the crease.
Numerous different types of devices have been proposed to increase transverse tension in a web. Such devices include bowed and flexible ("Mt. Hope") rollers, concave rollers, convex rollers, undercut rollers, spirally-grooved rollers, and deformable rollers. Each of these devices requires good frictional contact between web and roller to develop transverse tension. Inevitably, however, a roller which is sufficiently aggressive to remove moderate wrinkles by inducing axial separation of the web edges will induce axial slipping and skidding between the web and roller when there are few or no wrinkles to remove. Further, as web speed is increased, the air film carried into the nip between web and roller causes the web to float on the roller, destroying the roller's ability to spread the web edges. This occurs at web speeds well below 300 m/min for web tensions of about 2.5 Newtons per centimeter of web width, and it is especially a problem for thin webs which must operate at tension levels such as 1 N per centimeter of web width, or for ultra-thin webs which must operate at tension levels such as 0.20 N per centimeter of web width, or less. By "thin" is meant webs having thicknesses less than 0.025 mm. By "ultra-thin" is meant webs having thicknesses less than 0.006 mm (6 .mu.m). Another limitation of these rollers is that the energy to cause the spreading action in the web must come from the web itself if the rollers are not driven rollers. These energy losses are manifested as tension losses through the machine. Maintaining a constant tension level of, for example, 1 N/cm throughout the machine, requires that most, if not all, rollers be driven at web speed. This is impractical in many machines, some of which may utilize a thousand or more rollers.
It is an object of the invention to provide an improved method for smoothing tension-induced wrinkles from thin webs being conveyed.
It is a further object of the invention to provide improved apparatus for smoothing tension-induced wrinkles from thin webs being conveyed.
It is a still further object of the invention to provide an improved method for preventing creasing of thin webs being conveyed on contact rollers.
It is a still further object of the invention to provide improved apparatus for preventing creasing of thin webs being conveyed on contact rollers.
It is a still further object of the invention to provide an improved method for preventing creasing of thin webs being wound up.
It is a still further object of the invention to provide improved apparatus for preventing creasing of thin webs being wound up.